Catalytic reducing decomposition of oxides of nitrogen

ABSTRACT

The oxides of nitrogen in an exhaust gas are reductively decomposed over a catalyst of cerium oxide or uranium oxide in the presence of ammonia.

United States Patent 11 1 Matsushita et al.

[ 1 May 20, 1975 CATALYTIC REDUCING DECOMPOSITION OF OXIDES OF NITROGEN [75] Inventors: Kunichi Matsushita, Kawasaki;

Hikaru Sakurada, Yokohama; Kazuhiko Onuma, Kawasaki, all of Japan [73] Assignee: Mitsubishi Chemical Industries,

Ltd., Japan 221 Filed: Dec. 26, 1973 211 App]. NO.: 428,222

[30] Foreign Application Priority Data Dec. 25, 1972 Japan 48-784 [52] US. Cl. 423/2132; 60/301; 423/239 [5 1] Int. Cl. B0ld 53/00 [58] Field of Search 423/239, 213.2; 60/300 [56] References Cited UNlTED STATES PATENTS 3,008 796 ll/l96l Anderson et al. 423/235 3.l40.l48 7/1964 Hofer et al 423/2132 3,449,063 6/1969 Grifi'mg et al. 423/2l3.2 3,649,169 3/1972 Nicklin et al. 423/244 X FOREIGN PATENTS OR APPLlCATlONS l,259.298 l/l968 Germany 423/239 l,4l2,7l3 8/1965 France 423/239 Primary ExaminerG. O. Peters Attorney, Agent or Firm-Oblon, Fisher, Spivak, McClelland & Maier [57] ABSTRACT The oxides of nitrogen in an exhaust gas are reductively decomposed over a catalyst of cerium oxide or uranium oxide in the presence of ammonia.

11 Claims, No Drawings CATALYTIC REDUCING DECOMPOSITION OF OXIDES OF NITROGEN BACKGROUND OF THE INVENTION begin to decrease thus, there is a very demanding need for efficient methods of removing NO, because they cause photochemical smog by their oxidizing utilities. It is most important, therefore, that nitrogen oxides be removed from exhaust gases from polluting sources such as power plants, nitric acid plants, automobiles, and the like.

The conventional methods for removing NO, heretofore, have included scrubbing, absorption, and catalytic methods. The various types of catalytic methods include those in which an exhaust gas is contactedwith a catalyst and those in which an exhaust gas is contacted with a catalyst and a reducing agent. It has been difficult to obtain satisfactory results with scrubbing and absorption methods. In the catalytic methods in which no reducing agents are used, platinum group metals, oxides of transition metals, oxides of rare earth metals and the like have been used as catalysts. With this type of catalytic method, however, it has been difficult to obtain effective results without using temperatures greater than 600C.

On the other hand, the catalytic reducing decomposition methods it has been known to use platinum group metal catalysts, and hydrogen, carbon monoxide, natural gas hydrocarbons or liquid petroleum gas as a reducing agent. The catalytic reducing methods have certain advantage compared to other conventional methods. I

Usually exhaust gases contain from 0.5-l vol oxygen. For example, exhaust gases discharged from automobiles contain 0.5 vol oxygen, and exhaust gases discharged from nitric acid plants contain 3 vol% oxygen. Accordingly, the reducing agent present in the exhaust gases is exhausted by reaction with oxygen to complete combustion. In order to remove nitrogen oxides then, excess amounts of the reducing agent are required. If the concentration of oxygen is high, it is difficult to remove the heat generated by the reaction of the reducing agent with oxygen. This has been a serious drawback with the catalytic reduction method.

Another catalytic reduction method which is known involves the use of platinum as a catalyst and ammonia as a reducing agent. In the method, it has been difficult to prevent the side reaction of ammonia with oxygen. Further, more than half of the oxides of nitrogen are not reduced to nitrogen, but form N 0.

A need, therefore, continues to exist for a catalytic reduction method which removes nitrogen oxides from exhaust gases by converting the nitrogen oxides to nitrogen.

SUMMARY OF THE INVENTION Accordingly, one-object of the present invention is to provide a catalytic method for the reductive decomposition of oxides of nitrogen at high nitrogen oxide removal rates.

Another object of the invention is to provide a catalytic method for the reductive decomposition of oxides of nitrogen in exhaust gases with high activity and without substantial reaction between ammonia and oxygen.

Briefly, these objects and other objects of the invention as hereinafter will become more readily apparent can be attained by the reduction of oxides of'nitrogen in exhaust gases over a catalyst of cerium oxide or uranium oxide in the presence of ammonia.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The catalytic components can be supported on a carrier, if necessary. Suitable carries include alumina, silica, silica-alumina, zirconia, titania, cordierite, mullite, diatomaceous earth, silicon carbide, and the like. Preferably a-alumina is used. It is especially preferably to use a carrier which is prepared by calcining y-alumina at 550-l00OC. to yield an alumina carrier which is partially converted to a, 5 or O-alumina, from the viewpoint of strength and other properties.

The shape and size of the carrierused call bechojsen depending upon the applications intended. Various sizes of spherical, pellet and honeycomb shaped carriers can be used. The honeycomb type carrier has the advantage that it provides a system that has little pres sure loss, even when the flow velocity, i.e. the space velocity of the mixture gas of the exhaust gas and ammonia is high. Honeycomb carriers include those having guadrangular, triangular and circular sectional shapes. It is also possible to use those having a sinusoidal wave, spiders web or spiral shape.

Preferable carriers also include ceramics such as mullite, cordierite, and a-alumina coated with y-alumina of a suitable size. In order to coat the ceramic base it is immersed in a slurry, suspension or colloidal solution of 'y-alumina, or boehmite or aluminum hydroxide (which can be converted to 'y-alumina by calcination) and then isolated and calcined.

' Another preferred carrier is one in which the ceramic base is coated with titanium oxide. In order to coat the base with titanium oxide, a base is immersed in a solution of titanium tetrachloride, titanium trichloride or titanium sulfate for a predetermined time and is then isolated and kept in air or is treated with ammonia, whereby a coat of titanium oxide is formed by oxidation of hydrolysis of the titanium salt.

When the titanium tetrachloride is used, the complete conversion to titanium oxide is indicated by the lack of white smoke which is generated upon hydrolysis. The product is then washed with water at room temperature and if necessary, is calcined.

The acid treatment of a carrier with a mineral acid such as sulfuric acid, nitric acid, hydrochloric acid or an organic acid such as acetic acid before applying the catalytic component is very effective in improving the activity of the catalyst.

In order to perform the acid treatment, the carrier is immersed in an acid at l00C, preferably 20-60C for 1-5 hours. The time of the acid treatment is dependent upon the concentration of acid and the temperature.

It is possible to use cerium oxides or uranium oxides without using a carrier and have sufficient catalytic activity. However it is preferable to support the catalytic component on the carrier by a conventional method such as for example, by immersing the carrier in a solution of a cerium salt or a uranium slat on by spraying a solution of a cerium salt or a uranium salt onto the carrier.

Suitable cerium salts include cerium nitrate, cerium chloride, cerium sulfate, cerium ammonium nitrate or mixtures thereof.

It is not necessary to use cerium of high purity. It is possible to use cerium salts with another rare earth salt.

The carrier impregnated with the cerium salt is gradually heated in air or a nitrogen atmosphere until dry and then is calcined at 400700C to yield a catalyst coated with cerium oxide. The cerium oxide is most effectively supported by the carrier, in amounts ranging from 0.l-50% by weight, preferably l-30% by weight based on the total weight of the catalyst, i.e. the catalyst component and the carrier.

In the preparation of the catalyst, it is preferable to wash the carrier, which impregnates the cerium salt into the carrier, with water, aqueous ammonia or an aqueous solution of an ammonium salt such as ammonium carbonate, ammonium formate, ammonium acetate, or the like prior to drying and calcination of the catalyst. The concentration of the ammonia solution or the aqueous solution of ammonium salt used for washing usually. has a normality ranging from 1-20. The washing treatment has a normality ranging from l-20. The washing treatment is conducted at temperatures less than 100C, preferably at about room temperature for minutes to 2 hours. For example, the carrier can be washed for less than 1 hours in a quiescent solution at temperatures higher than 50C, or while being stirred at room temperature.

In order to conduct the catalytic reductive decomposition of NO, in exhaust gases with the catalyst prepared by the method above the exhaust gas and ammonia gas is contacted with the catalyst. It is very effective to pass an exhaust gas through the catalyst zone at a space velocity of 1,000-150,000 hr preferably 5,000l00,000 hr at a temperature of 200650C, preferably 250-550C. More than equimolar amounts, preferably more than 1.5 mole of ammonia gas per mole of NO, is added to the exhaust gas.

In accordance with the process of the invention, substantially high reductive activities for NO, are found when the oxygen content of the exhaust gas is higher than 0.1 vol%, especially higher than 0.5 vol.% and the reduction products are nitrogen and water.

The catalyst of the present invention has excellent activity over broad ranges of temperature. That is, the catalyst of the present invention exhibits low variations in its catalytic activity while maintaining a high catalytic activity. Thus, the catalyst can be used for various applications in many fields, and the catalytic reduction of NO, is significantly effective to prevent air pollution.

Having generally described this invention, a further understanding can be obtained by reference to certain specific examples which are provided herein for purposes of illustration only and are not intended tobe limiting unless otherwise specified.

In the examples, the concentration of NO was measured by the chemiluminescence method with a chemiluminescent NO analyzer (nitrogen oxide analyzer CLM-20l manufactured by Shimazu-seisaku-sho K.K.), and the NO, removal rate was calculated from the data showing the concentration of NO before and after the catalytic reductive decomposition.

EXAMPLE 1 Preparation of the Catalyst 1. In an aqueous solution of 6.54 g of cerium nitrate [Ce(NO 6H O] dissolved in 10 ml of deionized water, 10 ml of 'y-alumina particles having an average diameter of 5 mm was immersed for 1 hour, and the alumina was isolated and dried at. -1 10C for about 2-3 hours. The dried product was calcined at 250C for 1 hour, at 350C for 1 hour and then at 500C for 3 hours to yield a catalyst containing 10% by weight cerium oxide. This process was followed using 3.27 g of cerium nitrate, whereby a catalyst containing 5% by weight cerium oxide was obtained.

2. In an aqueous solution of 2.57 g of uranium nitrate UO (NO 6H O dissolved in 15 ml of deionized water, 13 m1 of 'y-alumina used in Example 1 was immersed for 1 hour. The particles were then isolate, and fired as above. By this procedure a catalyst containing 5% by weight of uranium oxide was obtained.

Catalytic Reduction of NO,

A gas mixture of 10% by volume oxygen, nitrogen and the NO gas mentioned in Table 1 wherein the remainder of the gas is nitrogen and NH gas was fed over a bed of the catalyst prepared above at a space velocity of 20,000 hour-l at the temperature shown in Table 1 to reduce the nitrogen oxides. The result are shown in Table 1.

TABLE 1 Experi- Metal Amount of Reaction added NO NO, NQ

ment oxide metal oxide tempera- NH before after removal No. supported ture decomdecomrate (w -7 (C) (pp pp p tion tion (pp p 1 CeO 10 395 3000 1940 645 66.8 385 3880 1940 625 67.8 440 3000 1940 730 62.5 2 CeO 5 320 2540 1950 915 53.0 360 2540 1950 830 57.5 420 2540 1950 830 57.5 400 3880 1950 750 61.5 3 U0 5 300 3000 2000 965 51.7 390 3000 2000 645 67.7 440 3000 2000 590 70.5 500 3000 2000 635 68.2 420 2100 2000 665 66.7

EXAMPLE 2 Preparation of the Catalyst A cylindrically shaped honeycomb (17 mm diameter, 25 mm height) catalyst support prepared by calcining 'y-alumina at 800C, was immersed in an aqueous solution of 5.669 g of cerium nitrate (Ce(NO 6H O) for 6 hours and isolated, dried and calcined in accordance with the process of Example 1. By this procedure a catalyst having a 6% by weight cerium oxide content was obtained.

Catalytic Reduction of NO,

A gas mixture containing 2000 ppm of NO 10% by volume oxygen, 3000 ppm of ammonia and nitrogen as the remainder, was passed through the catalyst prepared by the above process at a space velocity of 40,000 hr at the temperatures shown in Table 2 in order to reduce N0 The results are shown in Table 2.

TABLE 2 Reaction temperature (C) NO, removal rate (70) EXAMPLE 3 Preparation of the Catalyst 1. A cylindrically shaped honeycomb (17 mm diameter, 25 mm height) catalyst support made of oz-alumina coated with 'y-alumina was immersed in an aqueous solution of 9.067 g of cerium nitrate for 6 hours and isolated, dried and calcined in accordance with the process of Example 1. By this procedure a catalyst having 6% by weight cerium oxide was obtained.

2. A honeycomb support made of mullite coated with y-alumina which was prepared by immersing the mullite in a slurry of 'y-alumina and drying the isolated support (same size), was used to prepare a catalyst by the above procedure having 6% by weight cerium oxide.

3. A honeycomb support made of cordierite coated with y-alumina was used in the above process to yield a catalyst having 6% by weight cerium oxide.

Catalytic Reduction of NO,

In accordance with the procedure of Example 2, the catalytic reduction decomposition of NO, was conducted using the catalysts prepared by procedures l-3 above. The results are shown in Table 3.

EXAMPLE 4 Preparation of the Catalyst 1. A 15 ml quantity of 'y-alumina was immersed in 15 ml of titanium tetrachloride and isolated and oxidized in an atmosphere of air for several hours. The product was washed with water for several minutes and then dried by evaporation. The dried product was calcined at 500C for 3 hours in a flow of nitrogen. By this procedure, 15 ml of y-alumina coated with titanium oxide was obtained. A 15 ml quantity of the y-alumina coated with titanium oxide was immersed in 15 ml of an aqueous solution of9.34 g of cerium nitrate for 3 hours. The mixture was concentrated and dried at 500C for 3 hours. By this procedure an 'y-alu'mina substrate coated with 10% by weight of cerium oxide was obtained.

2. The procedure of Example 4-( l was followed except that a-alumina was used instead of 'y-alumina. By this procedure an a-alumina carrier coated with titanium oxide was obtained. A 15 ml amount of the a-alumina carrier coated with titanium oxide was immersed in 15 ml of aqueous solution of 15.2 g of cerium nitrate for 3 hours. The mixture obtained was dried and calcined under the same conditions of Examples 4-( l By this procedure a catalyst of a-alumina coated with titanium oxide and containing 10% by weight cerium oxide was obtained.

3. The procedure of Example 4-( 1) was followed except that silica having the same particle size diameter instead of 'y-alumina was used. A silica carrier coated with titanium oxide was obtained. A 15 ml amount of the silica carrier coated with titanium oxide was immersed in 15 ml of an aqueous solution of 6.583 g of cerium nitrate for 3 hours. The mixture obtained was dried and calcined under the same conditions of Example 4-( l By this procedures a catalyst of silica coated with titanium oxide and containing 10% by weight cerium oxide was obtained.

4. The procedure of Example 4-( l was followed except that silica-alumina having the same particle size diameter instead of 'y-alumina was used. A silicaalumina coated with titanium oxide was obtained. A 15 ml amount of the silica-alumina carrier coated with titanium oxide was immersed in 15 ml of an aqueous solution of 9.094 g of cerium nitrate for 3 hours. The mixture obtained was dried and calcined under the same conditions of Examples 4-( l By this procedure a catalyst of silica-alumina coated with titanium oxide and containing 5% by weight cerium oxide was obtained.

Catalytic Reduction of NO,

The procedure of Example 2 was followed for the catalytic reductive decomposition of NO, using the catalysts prepared in l-4 above.

The results are shown in Table 4.

TABLE 4 Continued Experiment Carrier Amount of Reaction N0,

N0. Catalyst Temperaremoval ture rate (g) 2 a-alumina 12.15 340 81.5 400 86.0 450 8 2.5 3 v silica 6.04 250 54.3 330 7 1.0 390 7 5.5 430 71.8 4 silica- 6.64 240 60.5 alumina 85.8 85.8 j 92.3 92.3 91 .0 91.0

EXAMPLE 5 Preparation of the Catalyst 1. A ml amount of 'y-alumina support was im- 'y-alumina c0ntained l0% by weight cerium oxide was mersed in conc. hydrochloric acid at room temperature for 1 hour. After separating the support from the hydrochloric acid, the product was washed with water at 50C for 1 hour and dried at 100C for 3 hours. The treated carrier was immersed in 15 ml of aqueoussolution of 15.2 g of cerium nitrate for 3 hours and was isolated and dried. The dried product was heated at 250C instead of hydrochloric acid. A15 ml amount of the treated carrier was immersed in 15 ml of an aqueous solution of 10.024 gof cerium nitrate for 3 hours. The

tions of Examples 5-( l By this procedure a catalyst of 'y-alumina containingl0% by weight cerium oxide was obtained.

. 3. The procedure of Example 5-(1) was followed except that 'y-alumina was treated with 10% nitric acid instead of hydrochloric acid. A 15 ml amount of the treated carrier was immersed in 15 ml of an aqueous solut-ionof 9.59 g of cerium nitrate for 3 hours. The

product obtained was calcined under the same condiobtained.

4. The procedure of Example 5-(1) was followed except that 'y-alumina was treated with 10% acetic acid instead of hydrochloric acid. A 15 ml amount of treated carrier was immersed in 15 ml of an'aqueous solution of 9.59 g of cerium nitrate for 3 hours. The product obtained was calcined under the same conditions of Example 5-( 1 By this procedure a catalyst of y-alumina contains 10% by weight cerium oxide was obtained.

5. The procedure of Example 5-( 1) was followed except that a-alumina was treated with 10% sulfuric acid instead of treating 'y-alumina with hydrochloric acid. A 15 ml amount of the treated a-alumina was immersed in 15ml of an aqueous solution of 14.7 g of cerium nitrate for 3 hours. The productobtained was calcined under the same conditions of Example 5-(1). By this procedure a catalyst of a-alumina containing 10% by weight cerium oxide was obtained.

6. The procedure of Example 5-(1) was followed except that diatomaceous earth was treated with 10% sul-' furic acid instead of a-alumina and hydrochloric acid. A 15 ml amount of the treated diatomaceous earth was immersed in 15 ml ofan aqueous solution of 3.74 g of cerium nitrate for 3 hours. The product obtained was calcined under the same conditions of Examples 5-(1). By this procedure a catalyst of diatomaceous earth containing 10% by weight cerium oxide was obtained.

7. The procedure of Example 5-( 1 was followed except that silicaalumina was treated with 10% sulfuric acid instead of a-alumina and hydrochloric acid. A 15 ml amount of the treated silica-alumina was immersed in 15 ml of an aqueous solution of 7.11 g of cerium nitrate for 3 hours. The product obtained was calcined under the same conditions of Example 5-(1). By this procedure a catalyst of silica-alumina containing 10% by weight cerium oxide was obtained.

Catalytic Reduction of NO,

The procedure of Example 2 was used for the catalytic reductive decomposition of NO, using the catalysts above. The results are shown in Table 5.

TABLE 5 Experimen Currier Treating Amount of Reaction N0,

N0. solution catalyst temperaremoval ture rate (8) l 'y alumina conc.HCl 7.33 310 61.5 360 80.5 460 91.5 540 88.0 2 y-alumina l07cl-l SO 6.15 250 53.3 4 310 83.3 360 93.8 440 95.3 510 93.5 3 y-alumina l0%HNO 6.77 320 67.5 380 82.3 460 87.3 520 84.3 4 -y alumina l0%CH -;COOH 7.1 l 310 61.8 390 84.3 500 90.8 550- 87.3 5 a-alumina 10%H SO l 1.43 250 58.5 310 70.5 380 74.8 420 71.0

6 diatomaceous earth. l0%H SO 3.68 270 68.5

TABLE 5 -Continued Experiment Carrier Treating Amount of Reaction N0,

N0. solution catalyst temperaremoval ture rate 8) 430 93.8 510 93.0 7 silicaalumina l7rH SO 5.63 260 57.5 320 77.5 400 85.5 450 83.0

EXAMPLE 6 I TABLE 6-Continued Preparation of the Catalyst 15 Experi- Carrier Ratei of Amqunt Reaction NO, I ment treatment cata yst o temperaremova 1. A 40 ml amount of 'y alumina carrier was 1m No suppom cmalyst ture rate mersed in 40 ml of an aqueous solution of 20.74 g of e o cerium nitrate for 1 hour. A 10 ml amount of the im- (g) (C) mersed alumina was washed with about 10 ml of water. 2 w 10 6,76 360 67,5 at room temperature for 10 minutes. The product was 0 mm) fig dried and heated at 250C for 1 hour, then at 350C for 500 7 :4 1 hour and calcined at 550C for 3 hours in a nitrogen 3 238 2 flow. By this procedure a catalyst of 'y-alumina contain- 440 :5 4 ammonium 5 6.76 360 67.5 mg by weight cerium oxide was obtained. format: 420 2. A 10 ml amount of an alumina carrier was 1m- (1 hour) 460 79.4 mersed in an aqueous solution of cerium nitrate. was w- 500 ashed 3 times with 10 ml'of water at room temperature for 10 minute intervals (total minutes). The product was calcined under the same conditions of Example 30 6-(1). By this procedure a catalyst of 'y-alumina con- EXAMPLE 7 taining 10% by weight cerium oxide was obtained.

3. A 15 ml amount of a y-alumina carrier was immersed in 15 ml of an aqueous solution of 3.684 g of cerium nitrate for 1 hour, and then was filtered. The product obtained was immediately immersed in 15 ml of ammonia (28% NH at 60C for 30 minutes, and then was washed with 15 ml of water of 60C. The product was dried at 90C for 2 hours, and then calcined under the same conditions of Example 6-(1). By this procedure a catalyst of 'y-alumina containing 5% by weight cerium oxide was obtained.

4. A 15 ml amount of a y-alumina carrier was immersed in an aqueous solution of cerium nitrate. Further, the carrier was immersed in 15 ml of an aqueous solution of 6.15 g of ammonium formate at room temperature for 1 hour and then was filtered and washed with water for 1 hour. The product obtained was dried and calcined under the same conditions of Example 6-(3). By this procedure a catalyst of 'y-alumina containing 5% by weight cerium oxide was obtained.

Catalytic Reduction of NO,

The procedure of Example 2 of the catalytic reductive decomposition of NO, was conducted using the catalysts of preparation l-4. The results are shown in Table 6.

TABLE 7 Reaction temperature. (C) NO, removal rate (7:)

Having now fully described the invention, it will be apparent to one of ordinary skill in the art that many changes and modifications can be made thereto without departing from the spirit or scope of the invention as set forth herein.

What is claimed as new and intended to be convere by Letters Patent is:

1. In a process for the catalytic reduction of oxides of nitrogen present in exhaust gas. the improvement which comprises: contacting said exhaust gas with a catalyst of cerium oxide or uranium oxide in the presence of ammonia.

2. The process of claim 1, wherein said catalyst of cerium oxide is supported on a carrier.

3. The process of claim 2, wherein said carrier is selected from the group consisting of alumina, silica, silica -alumina, titania, diatomaceous earth and zirconia.

with titanium oxide.

7. The process of claim 1, wherein said catalyst of cerium oxide is prepared by immersing a carrier treated with an acid in an aqueous solution of a cerium salt and then calcining said immersed cerium oxide.

8. The process of claim 1, wherein said catalyst of cerium oxide is prepared by immersing a carrier in an aqueous solution of a cerium salt; washing said immersed carrier in water, aqueous ammonia or an aqueous solution of an ammonium salt and than calcining said washed carrier.

9. The process of claim 1, wherein said catalyst contains 0.01 to 50% by weight cerium oxide or uranium oxide supported on a carrier.

10. The process of claim 1, wherein said exhaust gas containing oxides of nitrogen and ammonia is contacted with said catalyst at 200 to 650C.

11. The process of claim 1, wherein said exhaust gas containing oxides of nitrogen and ammonia is passed through a catalyst zone at a space velocity of 1000 to 150,000 hr 

1. IN A PROCESS FOR THE CATALYTIC OF OXIDES OF NITROGEN PRESENT IN EXHAUST GAS, THE IMPROVEMENT WHICH COMPRISES: CONTACTING SAID EXHAUST GAS WITH A CATALYST OF CERIUM OXIDE OR URANIUM OXIDE IN THE PRESENCE OF AMMONIA.
 2. The process of claim 1, wherein said catalyst of cerium oxide is supported on a carrier.
 3. The process of claim 2, wherein said carrier is selected from the group consisting of alumina, silica, silica-alumina, titania, diatomaceous earth and zirconia.
 4. The process of claim 1, wherein said catalyst of cerium oxide supported on a carrier is fabricated from gamma -alumina which is calcined at 550*- 1000*C.
 5. The process of claim 1, wherein said catalyst of cerium oxide is supported on a carrier of a base coated with gamma -alumina.
 6. The process of claim 1, wherein said catalyst of cerium oxide is supported on a carrier of a base coated with titanium oxide.
 7. The process of claim 1, wherein said catalyst of cerium oxide is prepared by immersing a carrier treated with an acid in an aqueous solution of a cerium salt and then calcining said immersed cerium oxide.
 8. The process of claim 1, wherein said catalyst of cerium oxide is prepared by immersing a carrier in an aqueous solution of a cerium salt; washing said immersed carrier in water, aqueous ammonia or an aqueous solution of an ammonium salt and than calcining said washed carrier.
 9. The process of claim 1, wherein said catalyst contains 0.01 to 50% by weight cerium oxide or uranium oxide supported on a carrier.
 10. The process of claim 1, wherein said exhaust gas containing oxides of nitrogen and ammonia is contacted with said catalyst at 200* to 650*C.
 11. The process of claim 1, wherein said exhaust gas containing oxides of nitrogen and ammonia is passed through a catalyst zone at a space velocity of 1000 to 150,000 hr
 1. 